When ethylene oxide is prepared by a silver catalyzed, vapor phase, partial oxidation of ethylene with molecular oxygen, a gaseous reaction effluent is obtained. This effluent can be extremely dilute with respect to the desired ethylene oxide product, containing, for example, from about 0.3 mole percent to about 5 mole percent of the desired material.
Since the ethylene oxide effluent is very dilute in the gaseous mixture produced from the oxidation reaction, this mixture is subsequently subjected to various treatments (absorptions, distillations, flash, and the like) in order to obtain pure ethylene oxide. However, these treatments do not sufficiently deal with low concentrations (e.g., concentrations measured in mole parts per million (ppm)) of formaldehyde impurities which can be present in the ethylene oxide sought to be purified.
For example, in some processes, formaldehyde can be removed as an overhead stream following condensation of the ethylene oxide from a stripping column, however, this has several disadvantages. If the formaldehyde concentration in the overhead stream is high, a solid paraformaldehyde phase can form in the overhead system of the column which can result in blockage and erratic operation and can possibly require shutdown and cleanout. See, e.g., J. Frederic Walker, Formaldehyde, pgs. 140-163 (3d Ed. Reinhold Publishing Corp.). On the other hand, in this process, if the overhead stream contains a low formaldehyde concentration, the relative amount of ethylene oxide therein could be excessive, resulting in yield loss of desired purified material.